This invention relates to apparatus for recording and verifying handwriting, in particular signatures.
Piezo-electric films used for verification of signatures written with any common writing pen or stylus are taught in U.S. Pat. No. 4,234,868. The pressure-time profiles recorded with such a system are difficult to reproduce by a forger, since each person has his or her individual timed sequences of hand movements when writing his or her signature.
However, such pressure-time profiles, recorded with piezo-electric membranes and a single electrode above and below the membrane, are often adversely affected by voltages produced by changing airspaces between the film and its substrate. These voltages are superimposed on the desired pressure-dependent signal, and are caused by changes in capacitance of the film with respect to its shielding. The resulting voltages often obscure the desired signal produced by the strain in the piezo-electric film, and thus reduce the efficacy of such pressure-time profile correlation devices in commercial applications.
Furthermore there is a whole range of dynamic features, based on variations in speed and direction of the pen or stylus, which cannot be recorded by a device which merely captures the pressure-time profile. German patent application Nos. DE-A3 236 057 and DE-A3 239 099 describe piezo-electric sensor tablets which could be used to track the position of a pen and thus record the dynamic features of changes in speed and direction mentioned above. However, in order to accurately record the position of the pen, a large number of individually connected electrodes are required, each having a sensing amplifier, threshold detector and associated circuitry.